The present invention relates generally to the field of computed tomography (CT) imaging systems and specifically to source and detector configurations for stationary CT systems to facilitate measurement of more mathematically complete projection data for image reconstruction. A CT projection data set comprises projection measurements from a multitude of angular positions, or views, of the X-ray tube and detector relative to the patient or object being imaged. A set of mathematically complete projection data contains measurements that are sufficient to reconstruct the imaged volume without artifacts, within the constraints of the data acquisition system. Mathematical incompleteness can arise from a completely missing view of projection data, missing projection data within a portion of a view, or an inappropriate selection of geometrical imaging parameters such as the speed at which the patient or object traverses the gantry in a helical acquisition mode. It is essential that the projection data be mathematically complete, otherwise, it may be impossible to reconstruct image data with the fidelity required for a particular application.
Computed tomography is a technique which creates two-dimensional cross-sectional images or three-dimensional volumetric images of three-dimensional structures. Such tomographic techniques may be particularly useful for non-invasive imaging, such as for security screening, baggage and package examination, manufacturing quality control, and medical evaluation.
Conventional CT imaging systems may include a CT gantry and an examination table or conveyor for moving objects to be scanned into and out of the imaging volume defined by the X-ray collimators within the gantry. In such systems, the gantry is typically a moveable frame that contains an X-ray source, which is typically an X-ray tube including collimators and filters on one side, and detectors with an associated data acquisition system (DAS) on an opposite side. The gantry typically also includes rotational components requiring slip-ring systems and all associated electronics, such as gantry angulation motors and positioning laser lights.
For example, in so-called “third generation” CT systems the X-ray source and the detector array are in a fixed arrangement that is rotated by the gantry within an imaging plane and around the object to be imaged, so that the angle at which the X-rays intersect the object constantly changes. An X-ray detector may include a crystal or ionizing gas that, when struck by X-ray photons, produces light or electrical energy that may be detected and acquired for generation of the desired images. Such rotational CT systems have limitations regarding rotational speeds, mechanical balancing of the systems, and power and thermal requirements that become increasingly complex due to the need for rotationally compliant components. Further these limitations constrain the possible rotational speed of the gantry, making such rotational systems unsuitable for applications requiring good temporal resolution or high throughput.
Other types of CT architectures are non-rotational, i.e., stationary, and include configurations that offer high scanning speeds. For example, in one such stationary CT system, both the X-ray source and the detector are stationary and encircle the imaging volume. In such a system, the X-ray source may be a distributed X-ray source comprising many discrete electron emitters along its length and a distributed anode.
Since both the X-ray source and detector are stationary in such stationary CT configurations, they need to be designed to facilitate appropriate scanning protocols. For example, in one possible axial scanning configuration, the distributed X-ray sources at both longitudinal extents of a centered detector may be slightly offset (vertically and/or radially) relative to the area detector array. As a result, a volume in the center of the field of view of the imaging system is not subjected to X-rays, prohibiting reconstruction in this volume. Likewise, in a helical scanning configuration, a distributed X-ray source may be placed between two area detectors that circle the entire imaging volume. The X-rays are emitted through a gap between the two detector arrays to administer X-ray flux to the imaging volume. Because the X-ray source is also distributed around the entire bore of the gantry, the gap encircles the entire imaging volume, which prevents measurement of mathematically complete CT projection data and artifact-free image reconstruction of the volume. For example, for a helical acquisition, every reconstructed slice has some missing projection data. As a result, the acquired projection data is mathematically incomplete.
It is therefore desirable to provide improved source and detector configurations or modified data acquisition protocols for stationary CT systems to facilitate measurement of more mathematically complete data for image reconstruction and to provide suitable algorithms for reconstructing data acquired by such techniques.